Methods for fabricating protective coating systems for gas turbine engine applications

ABSTRACT

Methods for fabricating protective coating systems for gas turbine engine applications are provided. An exemplary method of applying a protective coating to a substrate includes the steps of providing a substrate formed of a ceramic matrix composite material, forming a first coating layer directly on to the substrate and comprising an oxygen barrier material, a compliance material, or a bonding material and forming a second coating layer directly on to the first coating layer and comprising a thermal barrier material. The method optionally includes forming a third coating layer partially directly on to the second coating layer and partially within at least some of the plurality of pores of the second coating layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is a divisional of U.S. patent application Ser. No.14/156,502, filed on Jan. 16, 2014, the contents of which are hereinincorporated by reference in their entirety.

TECHNICAL FIELD

The present disclosure generally relates to methods for fabricatingprotective coatings for gas turbine engine applications.

BACKGROUND

Turbine engines are used as the primary power source for various kindsof aircraft and other vehicles. The engines may also serve as auxiliarypower sources that drive air compressors, hydraulic pumps, andindustrial electrical power generators. Most turbine engines generallyfollow the same basic power generation procedure. Compressed air ismixed with fuel and burned, and the expanding hot combustion gases aredirected against stationary turbine vanes in the engine. The vanes turnthe high velocity gas flow partially sideways to impinge onto turbineblades mounted on a rotatable turbine disk. The force of the impinginggas causes the turbine disk to spin at high speed. Jet propulsionengines use the power created by the rotating turbine disk to draw moreair into the engine, and the high velocity combustion gas is passed outof the gas turbine aft end to create forward thrust. Other engines usethis power to turn one or more propellers, electrical generators, orother devices.

Both airfoils and combustors made from silicon nitride or siliconcarbide have the potential to appreciably increase the operatingtemperatures of turbine engines. The high temperature and high pressureenvironment of the turbine engine as well as the high gas velocity cancause erosion of silicon based ceramics. The mechanism of some of theerosion loss is due to the formation of SiO₂ and SiO gas. Typically,combustion gas environments, including turbine engines, contain about10% water vapor. Oxygen containing water in the turbine reacts withsilicon nitride and silicon carbide to form silica scale on siliconbased ceramic surfaces. Water vapor can also react with the silica scaleto form silicon hydroxide, which is volatile. Evaporation of siliconhydroxide from ceramic surfaces and erosion of ceramic caused by highspeed combustion gases passing over ceramic surfaces leads to the lossof ceramic material from ceramic combustor and turbine components atrates of a few microns per hour.

U.S. Pat. No. 6,159,553 and US 2002/0136835 A1 show protective ceramiccoatings. Tantalum oxide alloyed with lanthanum oxide provides anenvironmental coating (EBC). However, tantalum oxide permits diffusionof oxygen, resulting in the formation of a SiO2 layer below the tantalumoxide layer. Published U.S. patent application 2002/0098391 by Tanaka etal discloses the use of rare earth silicates to form a protectivecoating to a silicon based substrate ceramic material. But the processdisclosed by Tanaka limits the coating composition because it allowsinteraction of the coating with the substrate.

Accordingly, there is a need for an improved coating and method to applythe coating for a high temperature (>2200° F. (>1200° C.)) barrierbetween an environmental coating and a substrate of silicon nitride orsilicon carbide. There is also a need for a diffusion coating that willprevent migration of cations out of a silicon-based substrate. There isas well a need to coat complex parts with a uniform dense oxidationresistant coating at a minimal cost. Furthermore, other desirablefeatures and characteristics of the present invention will becomeapparent from the subsequent detailed description of the invention andthe appended claims, taken in conjunction with the accompanying drawingsand this background of the invention.

BRIEF SUMMARY

In one embodiment, a method of applying a protective coating to asubstrate includes the steps of providing a substrate formed of aceramic matrix composite material, forming a first coating layerdirectly on to the substrate and comprising an oxygen barrier material,a compliance material, or a bonding material and forming a secondcoating layer directly on to the first coating layer and comprising athermal barrier material. The method optionally includes forming a thirdcoating layer partially directly on to the second coating layer andpartially within at least some of the plurality of pores of the secondcoating layer.

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the detaileddescription. This summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used as an aid in determining the scope of the claimed subjectmatter.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be described in conjunction withthe following drawing figures, wherein like numerals denote likeelements, and wherein:

FIG. 1 is an exemplary ceramic matrix composite turbine blade suitablefor use in a gas turbine engine and upon which the coatings of thepresent disclosure may be applied; and

FIGS. 2-6 illustrate, in cross section, coated turbine engine componentsand methods for fabricating coated turbine engine components inaccordance with various embodiments of the present disclosure.

DETAILED DESCRIPTION

The following detailed description of the invention is merely exemplaryin nature and is not intended to limit the invention or the applicationand uses of the invention. Furthermore, there is no intention to bebound by any theory presented in the preceding background of theinvention or the following detailed description of the invention.

Silicon carbide-silicon carbide matrix (“SiC—SiC”) materials arecurrently limited in operational use temperature by oxidation whichbegins around 2400° F., or even lower in some instances. While there aremany coating methods that have been put forth, all claiming to resolvethe issues of other methods, they each have issues of their own. Inother words, gaining a benefit in a property from one process ormaterial often leads to a shortfall in another property. The presentdisclosure provides a hybrid approach to creating an oxidation/thermalbarrier for SiC—SiC substrate materials to allow the use temperature tobe increased from about 2400° F. to about 3000° F. The hybrid approachemploys three or more layers each made using a differentprocess/composition/microstructure. The process for applying each layer,as well as the type/composition of each layer, are such that its strongpoints compliment the shortfalls of the others while creating a robustcoating.

In broad terms, with reference to the practice of the presentembodiments, a SiC—SiC substrate is appropriately cleaned/heat treatedfor a coating to be applied. First, a thin layer is applied to thesubstrate with the intent to create an oxygen barrier on the surface,and to decrease the surface roughness, enhance bonding, and/or providecompliance. The nature of the SiC—SiC substrate is such that the surfacehas many valleys and hills, as well as pits up to about 30 mil to about50 mil deep. A low viscosity process such as sol-gel is used topreferentially fill in the valleys while putting a thinner layer on thehills. Materials such as yttria-silicate and zirconia-silicate, forexample, may be used for this first layer. Additionally, materialsincluding an element selected from the group consisting of aluminum,zirconium, titanium, yttrium, hafnium, tantalum may be employed.Further, silicates of any of the foregoing may be employed. Afterprocessing of the first layer, a second layer is applied. This secondlayer is thicker and may have some porosity. One of the purposes of thesecond layer is as a thermal barrier, with the goal of adding about 100to about 600° F. to the operating temperature of the substrate material.The second layer may be made of an insulating material such as a rareearth silicate material. The thermal gradient created by the thermalbarrier layer can be customized by altering its composition, structure,and/or thickness. After processing of the second layer, a third layer isapplied. One purpose of the third layer is to fill/partially fill in theporosity of the second layer and/or coat the surface of the secondlayer, including the walls of the porosity. Application of the thirdlayer may be done via an infiltration process where the layer is drawninto the pores via capillary action or vacuum, or it may be done using aprocess such as aerosol deposition or other vapor process. In someembodiments, this third layer is not needed or does not need to beinfiltrated into the second layer.

Turning now to the Figures, FIG. 1 illustrates a ceramic matrixcomposite (CMC) blade 150 that is exemplary of the types of componentsor substrates that are used in turbine engines, although turbine bladescommonly have different shapes, dimensions and sizes depending on gasturbine engine models and applications. However, this invention is notrestricted to such substrates and may be utilized on many othersubstrates requiring thermal barrier protection, including othercomponents of gas turbine engines exposed to high temperature gases. Asnoted above, the blade 150 may be formed of a silicon carbidefiber/silicon carbide matrix composite material. The illustrated blade150 has an airfoil portion 152 including a pressure surface 153, anattachment or root portion 154, a leading edge 158 including a blade tip155, and a platform 156. The blade 150 may be formed with anon-illustrated outer shroud attached to the tip 155. The blade 150 mayhave non-illustrated internal air-cooling passages that remove heat fromthe turbine airfoil. After the internal air has absorbed heat from theSiC—SiC material, the air is discharged into a combustion gas flow paththrough passages 159 in the airfoil wall.

As generally known in the art, a SiC—SiC ceramic matrix compositematerial may include a SiC fiber-bonded ceramic or a SiC fiber-bondedceramic having a graded structure, for example. Regarding the SiCfiber-bonded ceramic, such a material may generally include inorganicfibers having mainly a sintered SiC structure, each of which contains0.01-1 wt. % of oxygen (O) and at least one or more metal atoms of metalatoms in Groups 2A, 3A, and 3B, and a 1-100 nm interfacial layercontaining carbon (C) as a main component formed between the fibers.Further, the SiC fiber-bonded ceramic having a graded structure maygenerally include a matrix, the matrix including inorganic fibers havingmainly a sintered SiC structure containing 0.01-1 wt. % of oxygen (O)and at least one or more metal atoms of metal atoms in Groups 2A, 3A,and 3B, and a 1-100 nm interfacial layer containing carbon (C) as a maincomponent formed between the fibers, a surface portion having a ceramicstructure including mainly SiC and being formed on at least part of thesurface of the matrix, a boundary portion interposed between the surfaceportion and the matrix and having a graded structure that changes fromthe structure of the matrix to the structure of the surface portiongradually and continuously.

These SiC—SiC materials include a volume fraction of about 90% or moreof SiC-based fibers. Such materials have high fracture toughness and areinsensitive to defects. The fiber material constituting the SiCfiber-bonded ceramic is mainly inorganic fibers that include a sinteringstructure containing mainly SiC, contain about 0.01-1 wt. % of oxygen(O) and at least one metal atom selected from the group including metalatoms in Groups 2A, 3A, and 3B, and are bonded very close to theclosest-packed structure. The inorganic fibers including a sintered SiCstructure include mainly a sintered polycrystalline n-SiC structure, orinclude crystalline particulates of β-SiC and C. In a region containinga fine crystal of carbon (C) and/or an extremely small amount of oxygen(O), where β-SiC crystal grains sinter together without grain boundarysecond phase interposed therebetween, a strong bond between SiC crystalscan be obtained.

FIGS. 2-6 illustrate, in cross section, coated turbine engine componentsand methods for fabricating coated turbine engine components inaccordance with various embodiments of the present disclosure. FIG. 2 isa cross-sectional view of a substrate 10 formed of a SiC—SiC material asdescribed above upon which is to be disposed a protective coating systemin accordance with an exemplary embodiment of the present disclosure.The substrate 10 may be employed for use in, for example, thefabrication of a turbine blade such as turbine blade 150 of FIG. 1. Asshown in FIG. 2, the substrate 10 has a generally irregular or “wavy”outer surface 11, including “pits” and “valleys,” upon which theprotective coating system is to be disposed, and which may be formed bythe woven fibers of the ceramic matrix composite. The irregular surfaceincludes deviations (+/−) from planar of several mils, such as about 1mil to about 5 mils. The outer surface 11 may also have larger defectsbeyond the illustrated irregular surface. These larger defects mayinclude deviations from planarity of about 30 mils to about 50 mils orgreater, in some instances.

With reference now to FIG. 3, to the outer surface 11 of the substrate10 is applied a first coating layer 12 that is provided in order tofill-in the pits and valleys, thereby minimizing the irregularity of theouter surface 11. The first coating layer 12 is also provided as anoxygen barrier between the substrate 10 and the surrounding environment.Still further, the first coating layer 12 may have the additionalproperties of promoting a bond between the substrate 10 andsubsequently-deposited layers, and a compliance layer for any possiblemis-match between the coefficient of thermal expansion (CTE) of thesubstrate 10 and subsequently-deposited layers. According to anexemplary embodiment, various sol gel coatings may be provided as thefirst coating layer 12. In a typical sol-gel process, the precursor issubjected to a series of hydrolysis and polymerization reactions to forma colloidal suspension. Once this occurs, the particles condense in anew phase, the gel, in which a solid macromolecule is immersed in asolvent. A variety of sol gels are known and a wide range of these maybe used in embodiments of the present coatings. For example, useful solgels may include, without limitation, sol gels of aluminum, zirconium,titanium, yttrium, hafnium, tantalum, and the like. Particularly, thesemetals, or the silicates of these metals, are suitable for use herein,as they provide excellent oxygen barrier properties. Exemplaryembodiments of barrier coatings may be formed from layered sol gelcoatings that include two or more coating layers formed one above theother. Each of the sol gel coating layers may be formed from the samesol gel, or some of the coating layers may be formed of a different solgel.

In general the sol gel single coatings of first coating layer 12 arethin, typically less than about 5 mils, such as less than about 3 mils.But multiple layers, each of the same or a different chemistry, maybuild up the total thickness of the sol gel barrier coating to about 20mils. The sol gel barrier coating may have a non-uniform coatingthickness so as to fill in the pits, valleys, and any defects. Forexample, as shown in FIG. 3, the thickness of coating layer 12 in region12A, which is a pit/valley region, is thicker than the thickness ofcoating layer 12 in region 12B, which is a “hill” region (betweenvalleys).

According to exemplary embodiments, sol gels may be applied to thesurfaces of a substrate by any of several techniques. For example, thinfilms of liquid sol gel can be applied to a portion of a substrate byspin-coating or dip-coating. Other methods include spraying, or rollcoating.

Referring now to FIG. 4, disposed over the first coating layer 12 is asecond coating layer 13. Second coating layer 13 is formed thicker thanfirst coating layer 12. For example, second coating layer 13 may beformed to a thickness from 10 mils to about 100 mils, for example fromabout 20 mils to about 50 mils. The second coating layer is provided asa thermal barrier coating to enhance the operating capabilities of a gasturbine engine component fabricated with the SiC—SiC substrate asdescribed above. For example, in one embodiment, the second coatinglayer is provided to a sufficient thickness such that an additionalabout 100 to about 600° F. of operating temperature is gained by thedeposition thereof (that is, the component is safely able to operate attemperatures of about 300° F. higher (or more) than it otherwise would).In particular embodiments, the addition of second coating layer 13raises the suitable operating temperature of a component formed withsubstrate 10 from about 2400° F. to about 2700° F. or greater, such asabout 3000° F. or greater.

The second coating layer 13 may generally include a rare-earth silicatematerial. Alternatively, aluminates, phosphates, and zirconates of therare earth elements may be used. As known in the art, rare earthelements include, among various others, strontium, lanthanum, yttrium,scandium, and others. For some embodiments, silicate of yttrium andscandium are particularly suitable. The second coating layer may beapplied over the first coating layer using any known methods. Thesemethods include, but are not limited to, plasma spraying, physical vapordeposition (PVD), and electron beam physical vapor deposition (EB-PVD),and dipping.

The material used for the second coating layer 13 and the method ofapplication thereof is selected such that the second coating layer 13has a porosity, as indicated by pores 13A. The pores 13A may be ofvarious shapes and sizes, as is known in the art. In some embodiments,the porosity of second coating layer 13 may be from about 10% to about70% (the percentage indicates the amount, by volume, of void space as aresult of the presence of pores 13A in layer 13). In other embodiments,the porosity may be from about 25% to about 50%. The number anddistribution of pores may be substantially equivalent throughout thethickness of layer 13. In other embodiments, deposition of secondcoating layer 13 may be provided such that there is a porosity gradientwithin layer 13. For example, a greater or lesser degree of porosity maybe provided in areas of layer 13 that are relative closer to layer 12,whereas a lesser or greater degree of porosity may be provided in areasof layer 13 that are relatively further from layer 12. As known in theart, greater porosity provides greater thermal barrier capabilities, butrenders the material less stable. Higher porosity will also likelycompromise the mechanical properties of the coating. As such, in oneembodiment, a relatively lesser porosity (for example from about 10% toabout 40%) is provided in areas of layer 13 that are relatively closerto layer 12, and a relatively greater porosity (for example from about40% to about 70%) is provided in areas of layer 13 that are relativelyfurther from layer 12.

The description of the exemplary method and coating continues withreference to FIG. 5, which illustrates the optional deposition of athird coating layer 14. Optional third coating layer 14 may be providedto fill in the porosity of the second layer and/or coat the surface ofthe second layer, including the walls of the porosity, orwholly/partially remain on the surface. The third coating layer 14 mayinclude, without limitation, materials including aluminum, zirconium,titanium, yttrium, hafnium, tantalum, and the like. Particularly, thesilicates, metallic, and phosphates of these metals are suitable for useherein.

FIG. 5 illustrates the third coating layer 14 deposited on top of thesecond coating layer 13. The layer 14 may be deposited to a thickness14A that is suitable to achieve the desired infiltration and fill of theporosity of layer 13, as will be described in greater detail below. Thethird coating layer 14 may be deposited by any of the above-describedmethods, such as spin-coating, dip-coating, spraying, roll coating,plasma spraying, PVD, EB-PVD, and others.

Referring now to FIG. 6, the layer 14 is caused to be infiltrated intothe porosity of the second layer 13. The embodiment shown in FIG. 6illustrates the material of layer 14 filling or at least coating all ofthe pores 13A in layer 13. However, in some embodiments, such completeinfiltration may not be possible or desired. For example, in embodimentswherein there is a gradient in pore size, with smaller pores beinglocated near layer 12, the material of layer 14 may only infiltrate acertain portion of the depth of layer 13 (namely into the pores 13Athereof. For example, in some embodiments, the material of layer 14 mayinfiltrate 70% or less of the thickness of layer 13, 50% of thethickness or less, or even 30% of the thickness or less. The amount ofmaterial of layer 14 deposited thus depends on the thickness and desiredfill properties of layer 13. For example, where complete fill of allpores 13A in the layer 13 is desired, a greater thickness of layer 14will be deposited. In a further example, where it may be only desirableto coat the walls of some of the pore 13A (i.e., not completely fill) ofsecond coating layer 13, a lesser thickness of layer 14 will bedeposited. Exact thickness for a given embodiment will ultimately bedetermined by the skilled artisan, but may generally be from about 5mils to about 20 mils, as initially deposited (FIG. 5).

Infiltration of the material of layer 14 into the layer 13 may beaccomplished in a variety of manners, including for example capillaryaction or an applied vacuum. In alternative embodiments, infiltration oflayer 13 may be accomplished using a process such as aerosol depositionor other vapor process. In some embodiments, the above-describedporosity gradient may be accomplished on the basis of how the infiltrantis processed, namely its quantity, the vacuum conditions, thetemperature, its viscosity, and the method of infiltration, among otherconsiderations. The infiltration can be performed in thegreen/wet/gaseous state, or it can be done during high temperatureprocessing. Upon infiltration of the material of layer 14 into layer 13,the thickness of layer 14 over layer 13 is substantially reduced, asindicated by reference numeral 14B. For example, in some embodiments,the final thickness of layer 14, after infiltration is performed, may beless than 5 mils, less than 3 mils, or less than 1 mil.

The coating system generally indicated in FIG. 6 by reference numeral 15is thus the protective coating formed according to the teachings of thepresent disclosure. Accordingly, protective coating systems for gasturbine engine applications and methods for fabricating such protectivecoating systems have been provided. The disclosed embodimentsbeneficially provide a hybrid approach to creating an oxidation barrierfor SiC—SiC substrate materials to allow the use temperature to beincreased from about 2400° F. to about 3000° F.

While at least one exemplary embodiment has been presented in theforegoing detailed description of the invention, it should beappreciated that a vast number of variations exist. It should also beappreciated that the exemplary embodiment or exemplary embodiments areonly examples, and are not intended to limit the scope, applicability,or configuration of the invention in any way. Rather, the foregoingdetailed description will provide those skilled in the art with aconvenient road map for implementing an exemplary embodiment of theinvention, it being understood that various changes may be made in thefunction and arrangement of elements described in an exemplaryembodiment without departing from the scope of the invention as setforth in the appended claims and their legal equivalents.

What is claimed is:
 1. A method of applying a protective coating to asubstrate includes the steps of: providing a substrate formed of aceramic matrix composite material; forming a first coating layerdirectly on to the substrate and comprising an oxygen barrier material,a compliance material, or a bonding material; and forming a secondcoating layer directly on to the first coating layer and comprising athermal barrier material.
 2. The method of claim 1, wherein forming thefirst coating layer is performed using a sol gel process and whereinforming the second layer is performed using plasma spraying, physicalvapor deposition (PVD), electron beam physical vapor deposition(EB-PVD), or dipping.
 3. The method of claim 1, further comprisingforming a third coating layer directly over the second coating layer. 4.The method of claim 3, further comprising infiltrating the third coatinglayer at least partially into the second coating layer.
 5. The method ofclaim 1, wherein the ceramic matrix composite material comprises asilicon carbide-silicon carbide (SiC—SiC) material.
 6. The method ofclaim 1, wherein the substrate comprises an irregular surface, andwherein the first coating layer is formed directly on to the irregularsurface.
 7. The method of claim 1, wherein the first coating layercomprises a material including an element selected from the groupconsisting of: aluminum, zirconium, titanium, yttrium, hafnium,tantalum.
 8. The method of claim 7, wherein the first coating layercomprises a metal silicate.
 9. The method of claim 1, wherein the firstcoating layer is formed with a variable thickness across the substrateof less than about 10 mils.
 10. The method of claim 1, wherein thesecond coating layer comprises a rare earth material.
 11. The method ofclaim 10, wherein the second coating layer comprises a silicate,aluminate, phosphate, or zirconate of a rare earth material.
 12. Themethod of claim 1, wherein the second coating layer has a thickness ofabout 10 mils to about 100 mils.
 13. The method of claim 1, wherein thesecond coating layer further comprises a plurality of pores such thatthe second coating layer comprises a porosity of about 10% to about 70%.14. The method of claim 13, wherein the porosity has a gradient withinthe second coating layer.
 15. The method of claim 13, further comprisingforming a third coating layer partially directly on to the secondcoating layer and partially within at least some of the plurality ofpores of the second coating layer.
 16. The method of claim 15, whereinthe third coating layer coats the walls of the pores but does notcompletely fill the pores.
 17. The method of claim 16, wherein the thirdcoating layer infiltrates the second coating layer to a depth of about70% or less of a total thickness of the second coating layer, therebycreating a porosity gradient in the second coating layer.
 18. The methodof claim 15, wherein the third coating layer is selected from the groupconsisting of: silicates, metallics, and phosphates of aluminum,silicon, zirconium, titanium, yttrium, hafnium, and tantalum.
 19. Amethod of applying a protective coating to a substrate includes thesteps of: providing a substrate formed of a ceramic matrix compositematerial, wherein the substrate has an exterior surface exhibiting adegree of valley/hill surface irregularity comprising a plurality ofhills and a plurality of valleys; forming a first coating layerdeposited from a sol-gel material over the substrate, wherein the firstcoating layer is characterized as an oxygen barrier material, acompliance material, and/or a bonding material, and wherein the firstcoating layer is formed directly on to the exterior surface of thesubstrate and conforms to the exterior surface of the substrate suchthat the first coating layer has a non-uniform coating thickness overthe substrate wherein the first coating layer has a thickness within theplurality of valleys that is greater than its thickness over theplurality of hills; forming a second, porous coating layer directly onto the exterior surface of the first coating layer and characterized asa thermal barrier material, wherein the second coating layer furthercomprises a plurality of pores within the second coating layer; andforming a third coating layer partially directly on to an exteriorsurface of the second coating layer and partially within at least someof the plurality of pores within the second coating layer, wherein thethird coating layer coats walls of at least some of the plurality ofpores but does not completely fill the at least some of the plurality ofpores.
 20. A method of applying a protective coating to a substrateincludes the steps of: providing substrate formed of a ceramic matrixcomposite material, wherein the substrate has an exterior surfaceexhibiting a degree of valley/hill surface irregularity comprising aplurality of hills and a plurality of valleys; forming a first coatinglayer deposited from a sol-gel material over the substrate, wherein thefirst coating layer is characterized as an oxygen barrier material, acompliance material, and/or a bonding material, and wherein the firstcoating layer is formed directly on to the exterior surface of thesubstrate and conforms to the exterior surface of the substrate suchthat the first coating layer has a non-uniform coating thickness overthe substrate wherein the first coating layer has a thickness within theplurality of valleys that is greater than its thickness over theplurality of hills; forming a second, porous coating layer directly onto the exterior surface of the first coating layer and characterized asa thermal barrier material, wherein the second coating layer furthercomprises a plurality of pores within the second coating layer such thatthe second coating layer comprises a porosity of about 10% to about 70%,and wherein the second coating layer consists of a rare earth silicatematerial; and forming a third coating layer partially directly on to anexterior surface of the second coating layer and partially within atleast some of the plurality of pores within the second coating layer,wherein the third coating layer coats walls of at least some of theplurality of pores but does not completely fill the at least some of theplurality of pores, and wherein the third coating layer infiltrates thesecond coating layer to a depth of 70% or less of a total thickness ofthe second coating layer, thereby creating a porosity gradient in thesecond coating layer, and wherein 30% or more of the total thickness ofthe second coating layer comprises pores that remain uncoated.